The subject invention is directed to the art of extrusion dies and, more particularly to an extrusion die of the type used for extrusion of synthetic resins and plastics.
The invention is particularly suited for use in an underwater type, plastic pelletizing apparatus and will be described with reference thereto; however, the invention is capable of broader application and could be adapted for use in other environments and for other purposes.
One of the more common apparatuses used for pelletizing plastic materials comprises a rotary extruder which extrudes the resin or plastic material in a molten or fluid state through an extrusion die having a multiplicity of small diameter extrusion passages. Typically, the face of the die is submerged in a cooling bath such that the plastic material solidifies just as it is leaving the die. As the material exits from the face of the die, it is cut into pellets or granules by a rotating knife passing over the die face.
A continuous and ongoing problem with the foregoing apparatus has been the difficulty of assuring that the material does not solidify during its passage through the die. As is apparent, premature solidification of the material can result in plugging or "freezing-off" of the extrusion passages and/or outlet orifices.
In an effort to maintain the material molten during its passage through the die, the dies have typically been designed with internal heating passages through which steam or heated oil has been circulated. In addition, various arrangements have been used to retard the flow of heat from the body of the die to the cooling bath. For example, layers of insulation have been placed over the exposed faces of the die, or between the die face and the main die body. Alternatively, the die passages have been provided with insulating sleeves of ceramic or the like to retard heat flow from the molten plastic to the die body or face.
While the above mentioned approaches have alleviated the difficulty with "freeze-off" to some extent, it still remains an ongoing problem. To that end, the subject invention provides an extrusion die construction which is relatively simple while operating to significantly reduce "freeze-off" and provide extended surface wear life.